Non-volatile memory devices are used in certain applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, i.e., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, and one or more insulators.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. These different structured states have different values of resistivity and therefore, each state can be determined by electrical sensing. Typical materials suitable for such applications include those utilizing various chalcogenide materials. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
The state change materials are also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed, as that value represents a physical state of the material (e.g., crystalline or amorphous). Further, reprogramming requires little energy to be provided and dissipated in the device. Thus, phase-change memory materials represent a significant improvement in non-volatile memory technology.
In addition to memory elements, switching elements, particularly fast switching devices, are desirable for a number of applications. Fast switching elements are capable of being switched between a relatively resistive state and a relatively conductive state and are useful as voltage clamping devices, surge suppression devices, and signal routing devices. Fast switching elements can also be used as access devices in memory arrays.
An important class of fast switching materials are the Ovonic Threshold Switch (“OTS”) materials. OTS materials, like many phase-change memory materials, typically include one or more chalcogen elements. Unlike phase-change memory materials, however, the compositions of OTS materials are such that no change in structural state occurs within the range of normal operation of the material. Instead, the OTS material retains an overall predominantly amorphous structure during operation. Application of a suitable energy signal, typically an electrical energy signal having a voltage above a critical threshold level, induces a change of the OTS material from a relatively resistive quiescent state to a relatively conductive transient state. The relatively conductive state persists for so long as the current passing through the OTS material remains above a critical holding level. Once the energy signal is removed or the current otherwise decreases below the level needed to sustain the relatively conductive state, the OTS material relaxes back to its relatively resistive quiescent state.
In an effort to improve scaling of the memory device or switching device to increase the density of memory arrays, one technique is to reduce the area of a bottom contact. Current methods to reduce bottom contact areas may be to create pores, or holes, through an insulator that expose the bottom contact. However, pores are difficult to manufacture in sublithographic dimensions. Moreover, sublithographic pores may be difficult to mass-produce consistently. Additionally, pore-exposed bottom electrodes typically require chemical vapor deposition (CVD) to achieve conformal filling of the pore with a phase-change or switching material and good contact with the bottom electrode. CVD methods may add to manufacturing time and costs.
Therefore, a need has arisen to produce an improved bottom electrode for a phase-change memory device. Moreover, it is desirable to reduce the manufacturing costs and process difficulties for producing the bottom contact, especially at dimensions approaching or entering the sublithographic regime.